The present invention generally relates to wireless communications systems, and more particularly relates to techniques for spatial precoding of signals transmitted from multiple antennas.
In several wireless communication systems, wireless receivers use one or more of several types of pilot signals to aid in demodulating the received signals. These pilot signals are commonly referred to as reference signals and/or reference symbols. In the 3rd-Generation Partnership Project (3GPP) specifications for the Long-Term Evolution (LTE) wireless system (also commonly referred to as the evolved UMTS Terrestrial Radio Access Network, or E-UTRAN), a receiving wireless device has two different pilot signal types to use for the data demodulation, common reference signals (CRS) and demodulation reference signals (DMRS). Details of these signals can be found in the 3GPP specifications, e.g., in “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical channels and modulation,” 3GPP TS 36.211, v. 10.4.0, December 2011 (available at www.3gpp.org).
CRS are pilots shared by all users, and are used for control channel (PDCCH) and data channel (PDSCH) demodulation as well as for signal measurements made for mobility decisions. DMRS, however, are intended for use by a single user, and thus directly correspond to data targeted to that particular user. DMRS was introduced in Release 9 of the LTE specifications; details can be found, e.g., in 3GPP TS 36.211 and 3GPP TS 36.213, which are available at www.3gpp.org.
The use of DMRS and multi-antenna transmission schemes make it possible for a sending network node to beam-form (pre-code) the transmitted pilot signals as well as the corresponding data signals, based on radio channel characteristics for the link between the transmitting node antennas and the receiver, so that optimized performance is achieved for that particular user. Precoding on the transmitter side is used to support spatial multiplexing and allows multiple signal streams to be transmitted simultaneously. This is achieved by applying a precoding matrix from a set of defined complex weighting matrices to the signal for combining data streams and mapping the combined data streams to multiple antennas for transmission.
The LTE standards continue to develop and are evolving more and more to the use of DMRS instead of CRS for estimating channel characteristics for demodulation purposes. Two main reasons for favoring DMRS over CRS are system performance improvements and coverage gains, which arise due to the possibility to dynamically optimize per-terminal performance based on the current radio channel characteristics.
Another reason for relying on demodulation pilots alone is that this approach raises the possibility that CRS can be removed completely from transmitted signals in a future release of the specifications. In systems operating according to Release 8 of the LTE specifications, CRS must be transmitted in every downlink subframe (see 3GPP TS36.211), regardless of whether or not there is any downlink data transmission in the subframe. One of the reasons for this “always-on” approach to CRS transmission is the need for idle mode terminals to be able to measure signal strength for cell selection. In releases up to at least Release 10 of the 3GPP standards, the wireless device or mobile terminal (user equipment, or UE, in 3GPP terminology) is able to choose which particular CRS to use for this purpose. As a result, the transmitting node (e.g., an evolved Node B, or eNB, in 3GPP terminology) does not know when mobile terminals are making mobility measurements, particularly if those mobile terminals are idle, and therefore cannot turn off CRS even if no actual downlink data transmission is ongoing.
FIG. 1 illustrates an LTE frame, including all of the signals that need to be transmitted from the network node regardless of load in the system, assuming LTE 3GPP Release 8. These signals include the Primary Sync Signal (PSS) and Secondary Sync Signal (SSS), which are found in subframes 0 and 5 of each LTE subframe, the Primary Broadcast Channel (PBCH), in subframe 0, and CRS, which are found at defined resource elements in all subframes.
As seen in the figure, the LTE frame includes 10 subframes. Each subframe consists of 14 Orthogonal Frequency Division Multiplexing (OFDM) symbols (12 when the long cyclic prefix is used), where the symbols are numbered from 0 to 13 (0 to 11 when a long cyclic prefix is used). CRS symbols, which are shown in the figure as solid black rectangles, are transmitted in OFDM symbols 0, 4, 7 and 11, on every sixth subcarrier. The SSS and PSS is transmitted in OFDM symbols 6 and 7, respectively, of subframes 0 and 5 of each LTE frame, in the central six resource blocks with respect to the system's frequency usage. The PBCH is also transmitted in several OFDM symbols in sub frame 0, again in the six central RBs.
One motivation for removing the requirement for “always-on” CRS transmission is that the base station/network node transmitter can be put into sleep mode in several, or even a majority, of the sub frames. This would allow the system to conserve energy in low-load scenarios. However, idle mode terminals need some known sync signals and CRS to rely on for synchronization to the network. This synchronization is needed for the mobile terminals to detect paging, and is also necessary for the mobile terminals to determine the proper timing for a random access transmission to a base station. However, the transmission of signals for these purposes could be done less frequently, in certain well-defined time/frequency positions that could be configured by the network node once a wireless device or mobile terminal registers with the network. Accordingly, in the discussions for the Release 11 (and onwards) for LTE, it has been proposed to only require transmitting network nodes to transmit CRS and other known data (sync symbols and broadcast messages) in a subset of the sub frames. Of course, the Primary Sync Signal and Secondary Sync Signal still need to be transmitted, for the wireless terminal to be able to do cell search and detect the cell. Since broadcast messages as well as paging signals are transmitted in sub frame 0 and 5, and since these data channels need some kind of reference signal for demodulation, there is a need to transmit sync signals and possible CRS at least in these sub frames. At a minimum, then, symbols that must be transmitted from a network node (assuming no load) are the synchronization signals and broadcast information, and possibly also some CRS in sub frames 0 and 5 for demodulation of broadcast and paging messages.
Hence, the removal of CRS gives the network (NW) node the ability to go into sleep mode in some or even a majority of the sub frames, in the event of no load or very low load. The resulting signal after this removal of CRS is called a “lean carrier” in the 3GPP standardization efforts. FIG. 2 illustrates two proposals for such lean-carrier solutions. The top of the figure, labeled “A,” represents a first proposed lean carrier structure. With this proposal, CRS are transmitted only in subframes 0 and 5, along with PBCH (subframe 0) and the sync signals PSS and SSS. The lower portion of the figure illustrates a second proposal for the lean carrier's structure, labeled “B,” in which no CRS at all are transmitted. In the latter case, PBCH detection at the terminal might rely on the PSS and SSS symbols. It will be appreciated that FIG. 2 illustrates two of the most extreme possibilities for a lean carrier. Lean carriers with CRS in one or more additional subframes are also possible, as are lean carrier structures in which CRS appear in only a subset of resource blocks in one or more subframes of each frame, or lean carrier structures in which CRS appear in a subset of subframes in a subset of frames. Lean carrier structures based on a combination of these features are also possible.
When a lean carrier is used, the transmitting network node need not transmit anything at all in some subframes (e.g., subframes other than 0 and 5), except when there is data to transmit. As discussed above, DMRS are preferably used when transmitting data, to optimize performance. However, in order to determine optimal pre-coding vectors (i.e., antenna-mapping weights used to apply phase and amplitude corrections to data and pilots at each of two or more antennas in a multi-antenna transmission mode) for optimized beamforming, the transmitting node needs to have knowledge of the propagation channel between the transmitting node antennas and the targeted mobile terminal. This is generally solved by letting the mobile terminal report channel state information (CSI), although other techniques for the network to learn the characteristics of the downlink channel are possible. In LTE, CSI is reported by the mobile terminal in the form of a precoder recommendation, which is based on channel measurements made by the mobile terminal. This precoder recommendation, which includes a Precoder Matrix Indicator (PMI), is based on channel measurements made by the mobile terminal and is used by the transmitting network node to determine the best pre-coding vector for transmitting data to the mobile terminal. CSI reporting might also be based on other formats, such as signaling that indicates a measured signal-to-noise ratio (SNR) or signal-to-noise-plus-interference ratio (SINR), channel rank information, etc. Thus, while the term CSI may sometimes be used herein to refer to LTE-specific reporting of channel state information, it should be understood more generally to refer to any data that characterizes, directly or indirectly, the propagation channel from the network node's transmitting antennas to a mobile terminal.
The effective use of beam-forming relies on good knowledge of the channel characteristics to select the optimal precoder for the downlink transmissions to the mobile terminal. However, good knowledge of the propagation channel can be difficult to obtain in high speed scenarios, due to rapid changes in the channel's characteristics. There is always a delay inherent in the CSI reporting to the network node (e.g., 3-10 milliseconds), and in the event of rapid channel changes, such as might occur when the mobile terminal is moving rapidly, the CSI information might be outdated by the time it is applied to the data. Accurate knowledge of the propagation channel may be unavailable in other circumstances as well, such as when a mobile terminal first goes into active mode and does not have accurate channel estimates, or when the SNR at the mobile terminal is very low, or when the mobile terminal is not configured to send precoding information to the base station. The wrong pre-coder can thus be applied in any of these circumstances, e.g., a precoder optimized not to the present radio channel, but the radio channel as it was several milliseconds earlier. Applying a precoder based on outdated CSI might actually make the receiving conditions worse, such that there is destructive interference between the radio signals from the multiple antennas as opposed to the constructive combining that is desired.
Under Release 10 of the LTE specifications, this problem can be solved by switching from a DMRS-based transmission method to CRS-based beam forming methods. This switching is accomplished through the use of Radio Resource Control (RRC) signaling sent to the mobile terminal to indicate which transmission mode to use, CRS-based or DMRS based. Details may be found in 3GPP TS 36.331, available at www.3gpp.org. Alternatively, the system may automatically fall back to a transmit diversity (TxDiv) scheme if the terminal is configured with no precoder feedback. CRS-based channel estimation is then used by the mobile terminal for demodulating data, which is transmitted to the mobile terminal using the TxDiv or large Cyclic Delay Diversity (CDD) approaches, which are both well-known in the art and described in 3GPP TS 36.211 and 3GPP TS 36.213.
However, these approaches to handling unreliable channel state information cannot be used effectively with a lean carrier, since no CRS is present in a majority of the downlink subframes. One possible solution to this problem is to simply turn on all CRS in a cell if it becomes known that the channel state information for a single mobile terminal is unreliable or unavailable. Clearly it will not be energy efficient to turn on the CRS to solve occasional problems with unreliable or non-existent precoder information for a single mobile terminal. Further, a solution based on turning on and off the CRS would require signaling to all UEs in the cell, to notify them of this CRS reconfiguration.
Hence, there is a need for improved techniques for managing DMRS-based and CRS-based transmissions in the event that reliable precoder information is unavailable, especially when techniques like lean carrier are used.